Just to note: Warriors&Warlocks (W&W) is M&M. It is not a separate system.
Yep- I knew that...sort of.
Also I don't see how a different system is going to change what people use. If they have spells they like or that work for them aren't they just as likely to gravitate toward the options in the new system that allow them to replicate those choices?
Along with the rules for designing spells will be somewhat of a return to 1Ed rules as written- you don't just get to pick your spells, you have to
learn them. This means Spellcraft checks.
So, right off the bat, the player will determine what aspects of magic the PC will study...but
dice rolls will determine his successes and failures (Spellcraft checks). With a successful check, the PC may just be able to make his own MM spell right away. OTOH, the PC may be
studying Force magic (to design a version of MM) intensely, but he may find certain aspects difficult to grasp, while simultaneously discovering a natural affinity towards charms. And after that, the PC will be able to continue to study and, who knows, maybe master that last difficult incantation or gesture that has been preventing him from making his own MM.
However, as they find scraps and pieces of scrolls and tomes, or study with other spellcasters, they'll get bonuses to learn those aspects of magic that others have learned before.
So if he had failed at learning a version of MM initially, but finds an arcanist who is competent in Force magic, he'll be able to try again, this time with bonuses.